Welding machines are known which have a system attached "in-line" for application of a thermoplastic material which applies a protective film to the longitudinal weld seam of metal tubes, in particular longitudinally welded tin and can sleeves. In principle there is a difference between application of a paint and a powder which is processed locally into a layer covering a seam.
Depending on machine type, the protective film is applied from above or below, where the metal tube to be processed is transported in the longitudinal direction of the application arm suspended on an application arm for application of the protective film from above, and in contrast standing on a conveyor belt for application from below.
In particular for the tinned metal cans used for the foodstuff industry, a clean sterilization-resistant internal covering of the weld seam with an uninterrupted protective film is of greatest importance. The tolerance to foodstuffs also requires the protective layer to resist hard mechanical processing such as the application of beads or edging. The application of the sterilization-resistant "hotmelt", as thermoplastic adhesives are known for short, has previously failed due to the fact also that these could not be applied with the technical means suitable for the conditions and could not withstand the said mechanical stresses.
EP, A1 0591107 describes a process for continuous application of protective coatings to cover a longitudinal weld seam of metal tubes. A hotmelt mass is guided continuously as a flexible hotmelt bead through the welding arm of a welding machine and deflected in the direction of the inside of the longitudinal weld seam of the metal tube. An application device presses the hotmelt bead onto a metal tube where it is melted and spread evenly over the width of the longitudinal weld seam. At the same time as the internal coating, an external protective coating can be applied to the longitudinal weld seam which allows elimination of the burning or drying oven. Although the welding and application arms with the coating device lie directly behind each other and the longitudinal weld seam of the continuous metal tube is still very hot at the application head, industrial practice has shown that melting the hotmelt bead on the longitudinal weld seam is problematical and no satisfactory results can be achieved.
The inventor has faced the task of developing a process and a device of the type described initially which in industrial operation guarantee problem-free economic application of the continuous protective film onto the internal longitudinal weld seam. In particular the continuous protective film should remain constant in thickness and form even in the area of the weld seam where the curvature fluctuates between successive tube bodies. The protective film should be structured with adjustable thickness over the width.